In quantum information systems, information is held in the “state” of a quantum system; typically this will be a two-level quantum system providing for a unit of quantum information called a quantum bit or “qubit”. Unlike classical digital states which are discrete, a qubit is not restricted to discrete states but can be in a superposition of two states at any given time.
Any two-level quantum system can be used for a qubit and several physical implementations have been realized including ones based on the polarization states of single photons, electron spin, nuclear spin, and the coherent state of light.
Quantum network connections provide for the communication of quantum information between remote end points. Potential uses of such connections include the networking of quantum computers, and “quantum key distribution” (QKD) in which a quantum channel and an authenticated (but not necessarily secret) classical channel with integrity are used to create shared, secret, random classical bits. Generally, the processes used to convey the quantum information over a quantum network connection provide degraded performance as the transmission distance increases thereby placing an upper limit between end points. Since in general it is not possible to copy a quantum state, the separation of endpoints cannot be increased by employing repeaters in the classical sense.
One way of transferring quantum information between two spaced locations uses the technique known as ‘quantum teleportation’. This makes uses of two entangled qubits, known as a Bell pair, situated at respective ones of the spaced locations; the term “entanglement” is also used in the present specification to refer to two entangled qubits The creation of such a distributed Bell pair is generally mediated by photons sent over an optical channel (for example, an optical waveguide such as optical fibre). Although this process is distance limited, where a respective qubit from two separate Bell Pairs are co-located, it is possible to combine (or ‘merge’) the Bell pairs by a local quantum operation effected, between the co-located qubits. This process, known as ‘entanglement swapping’, results in an entanglement between the two non co-located qubits of the Bell pairs while the co-located qubits cease to be entangled, at all.
The device hosting the co-located qubits and which performs the local quantum operation to merge the Bell pairs is called a “quantum repeater”. The basic role of a quantum repeater is to create a respective Bell pair with each of two neighbouring spaced nodes and then to merge the Bell pairs. By chaining multiple quantum repeaters, an end-to-end entanglement can be created between end points separated by any distance thereby permitting the transfer of quantum information between arbitrarily-spaced end points.
It may be noted that while QKD does not directly require entangled states, the creation of long-distance Bell pairs through the use of quantum repeaters facilitates long-distance QKD. Furthermore, most other applications of distributed quantum computation will use distributed Bell pairs.
The present invention is concerned with the creation of entanglement between spaced qubits and with the form, management and interaction of quantum repeaters to facilitate the creation of entanglements between remote end points.